1. Field of the Invention
The present invention relates generally to a diagnostic apparatus having an oximeter which is used for measuring a quantity of oxygen in brain. More particularly, the invention relates to a diagnostic apparatus wherein the changes of both arterial and venous blood volumes in brain are measured independently of each other, and the respective oxygen saturation rates in the brain are computed based on the measured data and displayed on a monitor screen.
2. Description of the Prior Art
In general, in diagnosing the function of a body organ, such as the cerebral tissues, the fundamental and important parameters to measure are the oxygen quantity in the body organ and the organ's utilization of oxygen. Supplying body organs with a sufficient quantity of oxygen is indispensable for the growth ability of fetuses and new-born infants. If the supply of oxygen to a fetus is insufficient, the probability that the fetus will not survive or that the new-born infant will die is high. Even if the newborn infant lives, the serious problems in the body organs may remain as sequelae. The insufficiency of oxygen affects every body organ, but especially causes a serious damage in the cerebral tissues
To quickly and readily measure a quantity of oxygen supplied to body organs, such as brain, a diagnostic apparatus using near infra-red spectroscopy technique has been developed as disclosed, for example, in U.S. Pat. No. 4,281,645 issued Aug. 4, 1981. This apparatus allows safe continuous monitoring of changes in blood and tissue oxygenation on an intact organ. This is made possible by observing spectral changes in the tissues caused by oxygenated haemoglobin, deoxygenated haemoglobin and cytochrome.
As shown in FIG. 3(a) , the absorption spectra of near infrared light (700 to 1300 nm) , .alpha.Hb.sub.2 and .alpha.Hb by oxyhaemoglobin (HbO.sub.2) and deoxyhaemoglobin (Hb) , respectively, are different from each other. And as shown in FIG. 3(b), the absorption spectra of .alpha.CyO.sub.2 and .alpha.Cy by oxidized cytochrome a, a.sub.3 (Cy), respectively, are different from each other. This diagnostic apparatus utilizes the above-described absorption spectra of near infrared light. Four near infrared light rays with different wavelengths, .lambda.1, .lambda.2, .lambda.3 and .lambda.4 (e.g. 775 nm, 800 nm, 825 nm, and 850 nm) are applied to one side of the patient's head with a time-sharing method and the transmission light rays from the opposite side of the head are in turn detected. By processing these four detected light rays with the prescribed calculation program, the density variations of oxyhaemoglobin (HbO.sub.2) deoxyhaemoglobin (Hb), oxidized cytochrome a, a.sub.3 (CyO.sub.2) and reduced cytochrome a, a.sub.3 (Cy) are calculated. These parameters, in turn, determine the variation of cerebral oxygen quantity.
FIG. 1 shows a system outline of a diagnostic apparatus 45. The apparatus 45 includes light sources such as laser diodes LD1 to LD4 which emit four near infrared light rays with different wavelengths of .lambda.1,.lambda.2, .lambda.3, and .lambda.4, respectively; a light source control device 55 which controls output timing of the light sources LD1 to LD4; optical fibers 50-1 to 50-4 which introduces near infrared light rays emitted by the light sources LD1 to LD4 to a patient's head 40; an illumination-side fixture 51 which bundles and holds end portions of the optical fibers 50-1 to 50-4; a detection-side fixture 52 which is fitted to the prescribed position of the opposite side of the patient's head 40; an optical fiber 53 which is held by the detection-side fixture 52 and introduces transmitted near infrared light from the patient's head 40; a transmission light detection device 54 which measures transmission quantity of near infrared light by counting photons of near infrared light introduced by the optical fiber 53; and a computer system 56 which controls the total diagnostic apparatus and determines the variation of oxygen quantity in cerebral tissues based on the transmission quantity of near infrared light.
The computer system 56 is equipped with a central processing unit (CPU) 62, a memory 63, output devices 64 such as a display and printer, and an input device 65 such as a keyboard, and these devices are connected to each other by a system bus 66. The light source control device 55 and the transmission light detection device 54 are connected to the system bus 66 as external I/O's.
The light source control device 55 receives instructions from the computer system 56 and drives the light sources LD1 to LD4 by respective driving signals ACT1 to ACT4 as shown in FIGS. 2(a) to 2(d). As shown therein, one measuring period M.sub.k (k=1, 2, . . . ) consists of N cycles of CY1 to CYn. At a phase .phi.n1 in an arbitrary cycle CYn, no light source of LD1 to LD4 is driven and therefore the patient's head 40 is not illuminated by the near infrared light from the light sources LD1 to LD4. At the phase .phi.n2 the light source LD1 is driven and the near infrared light with the wavelength of, for example, 775 nm is emitted from it. In the same manner, at the phase .phi.n3 the light source LD2 is driven and the near infrared light with the wavelength of, for example, 800 nm is emitted from it; at the phase .phi.n4 the light source LD3 is driven and the near infrared light with the wavelength of, for example, 825 nm is emitted from it; and at the phase .phi.n5 the light source LD4 is driven and the near infrared light with the wavelength of, for example, 850 nm is emitted from it. In this manner, the light source control device 55 drives the light sources LD1 to LD4 sequentially with a time-sharing method.
Referring again to FIG. 1, the transmission light detection device 54 is equipped with a filter 57 which adjusts the quantity of near infrared light outputted to lenses 70 and 71 from the optical fiber 53; a photomultiplier tube 58 which converts the light from the filter 57 into pulse current and outputs it; an amplifier 59 which amplifies the pulse current from the photo multiplier tube 58; an amplitude discriminator 60 which eliminates the pulse current from the amplifier 59 whose amplitude is smaller than the prescribed threshold value; a multichannel photon-counter 61 which detects photon frequency in every channel; a detection controller 68 which controls the temperature of a cooler 69 containing the photomultiplier tube 58.
To use the above-described diagnostic apparatus, the illumination-side fixture 51 and the detection-side fixture 52 are firmly fitted to the prescribed positions of the patient's head 40 by using tape or the like. Once fitted, the light sources LD1 to LD4 are driven by the light source control device 55 as shown in FIGS. 2(a) to 2(d), respectively, so that the four near infrared light rays with different wavelengths are emitted from the light sources LD1 to LD4 sequentially with the time-sharing method, and the light rays are introduced by the optical fibers 50-1 to 50-4 to the patient's head 40. As bones and soft tissues in the patient's head 40 are transparent to the near infrared light, the near infrared light is partially absorbed by haemoglobin in blood and cytochrome a, a3 in cells and outputted to the optical fiber 53. The optical fiber 53 introduces the light to the transmission light detection device 54. At the phase .phi.n1, no light source of LD1 to LD4 is driven, and therefore, the transmission light detection device 54 detects dark light.
The photomultiplier tube 58 in the transmission light detection device 54 is used with a photon-counting device that has high sensitivity and operates at high response speed. The output pulse current from the photomultiplier tube 58 is sent to the amplitude discriminator 60 through the amplifier 59. The amplitude discriminator 60 eliminates the noise component whose amplitude is smaller than the prescribed amplitude threshold and sends only the signal pulse to the multi-channel photon-counter 61. The multi-channel photon-counter 61 detects photons only in the periods T.sub.o. The periods T.sub.o are synchronized with the driving signals ACT1 to ACT4 for the respective light sources LD1 to LD4 as shown in FIG. 2(a) to 2(d) by a control signal CTL as shown in FIG. 2(e). The control signal CTL is generated by the detection controller 61. The multi-channel photo-counter then counts detected photon number of every light with each wavelength sent from the optical fiber 53. The transmission data of every infrared light with each wavelength are obtained through the above-described procedure.
As shown in FIGS. 2(a) to 2(e), at the phase .phi.n1 in the cycle CYn of light source control device 55 no light source of LD1 to LD4 is driven, therefore the dark light data d are counted by the transmission light detection device 54. At the phases .phi.n2 to .phi.n5 the light sources LD1 to LD4 are sequentially driven with the time-sharing method and the transmission light detection device 54 sequentially counts the transmission data t.sub..lambda.1, t.sub..lambda.2, t.sub..lambda.3 and t.sub..lambda.4 of the respective near infrared light rays with different wavelengths .lambda.1, .lambda.2, .lambda.3 and .lambda.4.
The counting of the dark light data d and the transmission data t.sub..lambda.1, t.sub..lambda.2, t.sub..lambda.3 and t.sub..lambda.4 which is sequentially performed in the cycle CYn, is continued N times from CY1 to CYn. That is, one measuring period M.sub.k (k=1,2, . . . ) includes N cycles. A concrete example is as follows; if one cycle is 200 .mu.sec and N is 10000, the measuring period Mk becomes 2 sec. At the time of finishing of one measuring period Mk, the counting result of the dark light data D ##EQU1## and the counting results of the transmission data T.sub..lambda.1, T.sub..lambda.2, T.sub..lambda.3 and T.sub..lambda.4 ##EQU2## are transferred to the computer system 56 and stored in the memory 63.
The processor 62 performs the subtraction of the dark light component by using the combination of the transmission data and the dark data (T.sub..lambda.1, T.sub..lambda.2, T.sub..lambda.3, T.sub..lambda.4, D) M.sub.k being stored in the memory 63 after one measuring period M.sub.k and the combination of those (T.sub..lambda.1, T.sub..lambda.2, T.sub..lambda.3, T.sub..lambda.4, D) .sub.Mo at the start of measuring, and calculates the variation rates of the transmission light .DELTA.T.sub..lambda.1, .DELTA.T.sub..lambda.2, .DELTA.T.sub..lambda.3 and .DELTA.T.sub..lambda.4. That is, the variation rates of the transmission light .DELTA.T.sub..lambda.1, .DELTA.T.sub..lambda.2, .DELTA.T.sub..lambda.3 and .DELTA.T.sub..lambda.4 are calculated as: EQU .DELTA.T.sub..lambda.j =log[(T.sub..lambda.j -D).sub.MK /(T.sub..lambda.j -D).sub.MO ] (1)
The use of logarithm in the above calculation of .DELTA.T.lambda.j is to express the variation as an optical density.
Using the above-calculated variation rates of the transmission light .DELTA.T.sub..lambda.1, .DELTA.T.sub..lambda.2, .DELTA.T.sub..lambda.3 and .DELTA.T.sub..lambda.4, density variations of oxyhaemoglobin (HbO2), deoxyhaemoglobin (Hb), oxidized cytochrome a, a.sub.3 (CyO.sub.2) and reduced cytochrome a, a.sub.3 which are expressed as .DELTA.X.sub.HbO2, .DELTA.X.sub.Hb, .DELTA.X.sub.CyO2 and .DELTA.X.sub.Cy, respectively, can be determined. That is, each of density variations of .DELTA.X.sub.HbO2 .DELTA.X.sub.Hb, .DELTA.X.sub.CyO2 and .DELTA.X.sub.Cy is calculated as: ##EQU3## where .alpha..sub.ij is an absorption coefficient of each component i(HbO.sub.2, Hb, CyO.sub.2, Cy) for each wavelength .lambda..sub.j (.lambda..sub.1, .lambda..sub.2, .lambda..sub.3, .lambda..sub.4) and is predetermined from FIGS. 3(a) and 3(b), and l is the length of the patient's head 40 along the traveling direction of the near infrared light.
As the above-detected density variation components, .DELTA.X.sub.HbO2, .DELTA.X.sub.Hb, .DELTA.XCyO.sub.2 and .DELTA.XCy, reflect the variation of oxygen quantity in the brain, the variation of oxygen quantity in the brain can be determined by outputting these detected results from the output device 64. The diagnosis thus is made based on these results.
The measured absorption spectra of near infrared light rays primarily dependent on haemoglobin and the absorption spectra is little affected by cytochrome. This is due to the fact that an amount of haemoglobin in body organ is several times as many as an amount of cytochrome and that change of cytochrome from oxydization to deoxydization or vice versa is small in amount in a normal condition because of cytochrome's oxygen affinity which is stronger than haemoglobin. Therefore, it is reasonably assumed that the change of oxygen quantity in body organ is substantially equivalent to the change of haemoglobin density in blood.
It is important in in clinical pathology and it has in fact long been required that data regarding the oxygen saturation rate of the blood flowing through a body organ be given to a clinician to perform diagnosing. However, the above-described apparatus is incapable of providing such data, although it is possible to measure and display the density variations of oxyhaemoglobin (HbO.sub.2) deoxyhaemoglobin (Hb) , and oxidized cytochrome (CyO.sub.2) and reduced cytochrome (Cy).
Here, the oxygen saturation rate of the increased or decreased blood .DELTA.SO.sub.2 to be obtained is defined by a ratio of the density variation of oxyhaemoglobin (HbO.sub.2) to a sum of the density variations of oxyhaemoglobin (HbO.sub.2) and deoxyhaemoglobin (Hb), i.e., EQU .DELTA.SO.sub.2 =K.times..DELTA.[X.sub.HbO2 ]/{.DELTA.[X.sub.Hb ]+.DELTA.[X.sub.HbO2 ]} (3)
where K is a predetermined constant, typically 100 for percentage representation.
One solution to the above problem is proposed in Japanese Laid-Open Patent Publication No. 63-275324 wherein an arterial oxygen saturation rate SaO.sub.2 in the brain is obtained by the computation of .DELTA.HbO.sub.2 /(.DELTA.HbO.sub.2 +.DELTA.Hb) upon measuring a total amount of haemoglobin (.DELTA.HbO.sub.2 +.DELTA.Hb) and an amount of oxyhaemoglobin (.DELTA.HbO.sub.2) which are modulated in synchronism with heartbeat. The heartbeat is pulsation of the heart coincident with ventricular systole.
According to this technique, while it is possible to provide the arterial oxygen saturation SaO.sub.2 which is representative of change in supply of oxygen to the brain, information regarding venous blood is not available which is representative of brain metabolism, utilization of oxygen in the brain or the like.
It has also been proposed to obtain the oxygen saturation rate with the use of near infrared spectroscopy technique, wherein the change of total volume of haemoglobin (.DELTA.HbO.sub.2 +.DELTA.Hb) is monitored at all times and computation of .DELTA.HbO.sub.2 /(.DELTA.HbO.sub.2 +.DELTA.Hb) is performed when the total volume of haemoglobin or the blood volume is changed for some reason more than a predetermined level. This technique is advantageous in that oxygen saturation rate is obtained resulting is from the change of blood volume which may occur when a newborn infant is applied with pressure caused by aspiration or maneuver at the time of birth or when the head portion of fetus is moved up and down. This technique is particularly advantageous in that information of other than the arterial status is available.
However, this technique is not applicable to adults or general diagnosing. Further, information regarding both the arterial and venous blood cannot be provided independently of each other.